The background of the invention will be discussed in two parts:
1. Field of the Invention
This invention relates to circuits for use with a multi-character or digit alphanumeric gas discharge display tube, and more particularly to a circuit for level shifting an anode driver input in such circuits.
2. Description of the Prior Art
In multi-character alphanumeric gas discharge display tubes, one such tube construction includes a plurality of character or digit locations, with each digit location being defined by seven (or more) cathode electrode segments and one common anode, selectively controlled by suitable logic for individually energizing the so-selected segments and anode to display a character, such as a digit. In multiplex operation of such displays, a given period of time is used to individually energize the anode of each digit location, with each digit location being energized for a fraction of the total time period. For example, in a five digit location alphanumeric display tube, each digit location will have the anode energized for approximately one-fifth of the total time, and at the end of this total time the cycle of energization of each digit location will be repeated at a frequency of repetition that is not discernable to the human eye so that each digit will appear to be constantly illuminated. During this "refresh" cycle the voltage levels of the segments are being altered to effect the energization of the proper segments in the addressed digit (anode) position.
In order to operate the gas tube display properly, the anodes of each digit location need to have an "on" potential of approximately 160 to 200 volts and an "off" potential of about 130 volts. The cathodes need an "off" potential of about 80 volts and an "on" potential of close to ground. The shifting of the levels of the anode and cathode potentials as required to operate the gas tube display are accomplished by use of the normal logic signals from a common logic generator which is also utilized to determine which of the electrode segments of each digit location are to be energized. The normal logic signals for these purposes for an "off" condition will be zero volts (or very near ground), and for the "on" logic signal the voltage will be, for example, approximately 15 volts.
A typical driving circuit for a plasma display panel or a gas discharge display tube is shown as described in U.S. Pat. No. 3,842,314 issued Oct. 15, 1974 to Iwakawa et al. The driving circuit shown in that patent is representative of a circuit utilized to energize the electrode segments in response to the logic signals.
In plasma discharge or gas discharge display tubes, since ionization is utilized, by having each digit location properly ionized at a given level by a keep-alive electrode, the individual digit location or cell can be kept from extinguishing as each cell is energized during the multiplexing operation. In a gas discharge tube the firing potential, or the potential necessary to discharge the cell (resulting in illumination) is higher than that necessary to maintain the discharge within the cell once fired. By utilization of a keep-alive electrode, ionized particles within the cell is shown and described in U.S. Pat. No. 3,801,862 entitled "Plasma Cell Voltage Control Circuit" issued Apr. 2, 1974 to Robert R. Skutt. The means for utilizing the keep-alive cell and the means for energizing it are shown and described in the aforesaid patent.
In a multiplexing operation, where each cell is energized after de-energization of the preceding cell, even with a keep-alive electrode, there is a problem in level shifting an anode driver input of a digit location or cell. Since large potentials are being driven by the use of the lower voltage logic potentials, such level shifting circuits tend to be complicated and require the use of electrical components having unnecessarily high voltage ratings.
The level shifting of the anode potentials, in prior art circuits, have employed means coupled within the anode driver circuit to effect the potential shifting required as the anode potential of each cell is varied during the multiplexing operation. Such means have included, for example, resistor/transistor or capacitance coupled shifters electrically connected between the anode of the cell and the voltage source, which as above mentioned, is utilized to effect an anode potential from an "off" potential of about 130 volts to an "on" potential of approximately 160 to 200 volts. Consequently, with either the resistance coupled shifter or the capacitance coupled shifter, the potentials being controlled are between approximately 130 and 200 volts, thus requiring electrical components that must withstand the maximum voltage being utilized. Other such shifter circuits have employed Zener diodes and gas discharge lamps, in both cases again, coupled in circuit between the anode of the cell and the voltage source. Similarly, the voltage breakdown requirements of the individual electrical components must be selected in accordance with the anode potential requirements.
On the other hand, the cathode drive signals are much lower in potential and as previously stated, the cathodes need an "off" potential of about 80 volts and an "on" potential of close to ground.
Accordingly, it is an object of this invention to provide a new and improved level shifting circuit.
It is another object of this invention to provide a new and improved gas discharge tube driver and level shifter circuit using the tube anode to cathode drop in the level shifting circuit instead of a high voltage transistor, or lower voltage transistor and capacitor.